The Baruntse liner will drop weight on the Spantik, fit better over all once molded and be more durable than the factory Spantik liner. Down side is it is less breathable.
When the Spantik first came out heat molding its liner was more a after thought than an intentional design feature. It shows with the tardy info on molding from La Sportiva, The Baruntse liner was designed to be heat molded from the beginning. A good shop will mold the inner for free and guarantee the fit.
I still use the Spantik and original Spantik liner. But I made the effort to mold mine which improved the fit as it does for most everyone.
Make a quick Internet search on the topic and check out the various Spantik "fit" threads on UKC.
The Spantik has become "THE" boot for cold weather climbing. It is literally the uniform boot for winter climbing in Cham. Few have used both boots enough to comment let alone recognise the difference in fit or the differing abilities of each on technical ground. If the Baruntse had been available first I suspect it would be a role reversal as to the popularity of each model.
The Baruntse and the 6000 being used on a one day ascent of the Cassin here:
http://vimeo.com/26046094